Is there a way to split a face in cubes (but remaining as one mesh) ? (explode modifier ?)
The frontmost image has used inset, extrude, and some faces between extrusions have been deleted.




Select, Inset, Extrude Individual Faces which can create faces targeted for deletion if you like. Blue sphere shows inset before any extrusion.  Yellow edit mode is the extrusion. Image above.




Inset skipped.  Image Above



In Edit Mode


Face Mode
Make a selection of faces
Inset Faces with the individual option ... See the tools keystrokes listed in the UI. Press I for individual inset [ON]
Menu ... Extrude Individual Faces .  Be careful to use the new selection provided by the tool.  If you loose the selection then use [undo] to recover the selection
Set the pivot for scaling to individual origins .. with the upper tool bar ... yellow arrow
Scale ... to scale the selected faces to the center of each face
Improve these steps





Delete Faces if you like to create islands
Know that you in edit mode you can add another shape such as a cube et cetera
You can always join two different meshes into one mesh